The present invention relates to an electromagnetic flowmeter which measures the flow rate of a fluid having a conductivity in various process systems and, more particularly, to a two-wire electromagnetic flowmeter which outputs a measurement value by adjusting an output current flowing to a pair of power supply lines that supply an external voltage from a DC power supply.
In a conventional two-wire electromagnetic flowmeter, an exciting coil is arranged such that the generation direction of a magnetic field becomes perpendicular to the flowing direction of a fluid that flows through a measurement tube. When a rectangular-wave-shaped exciting current Iex having a predetermined frequency is supplied to the exciting coil, a signal electromotive force (a signal proportional to the flow rate) is detected, which is obtained, in accordance with the Faraday's law, between electrodes that are arranged in the measurement tube to be perpendicular to the magnetic field generated by the exciting coil. A measurement value with respect to the maximum flow rate value is obtained as a 0% to 100% value by arithmetic processing by a CPU (Central Processing Unit) on the basis of the detected signal electromotive force. A current (output current) flowing to a pair of power supply lines that supply an external voltage to the two-wire electromagnetic flowmeter is adjusted within the current range of 4 to 20 mA in accordance with the obtained measurement value.
As shown in FIG. 5, an external voltage Vs from a DC power supply 200 is supplied to a conventional two-wire electromagnetic flowmeter 100 through a pair of power supply lines L1 and L2. An external load RL (resistance: 250 Ω) is connected to the power supply line L2 (DC 24 V). In this case, the value of the external voltage Vs supplied to the two-wire electromagnetic flowmeter 100 is obtained by subtracting the voltage drop at the external load RL from the power supply voltage of the DC power supply 200.
The conventional two-wire electromagnetic flowmeter 100 is constituted by an exciting coil 2 which is arranged such that the generation direction of a magnetic field becomes perpendicular to the flowing direction of a fluid that flows through a measurement tube 1, an exciting circuit 3 which generates an exciting voltage Vex between a first line LA and a second line LB and also periodically supplies the exciting current Iex having a rectangular waveform to the exciting coil 2, detection electrodes 4a and 4b which are arranged in the measurement tube 1 to be perpendicular to a magnetic field generated by the exciting coil 2, a ground electrode 5, and a flow measuring output circuit 6 which detects a signal electromotive force obtained between the detection electrodes 4a and 4b, obtains a measurement value on the basis of the detected signal electromotive force, and adjusts an output current I (Iout) to be returned to the DC power supply 200 within the current range of 4 to 20 mA in accordance with the obtained measurement value.
The exciting circuit 3 is constituted by an exciting voltage circuit (constant voltage circuit) 3-1, D/A circuit 3-2, and exciting current adjustment circuit 3-3. The exciting voltage circuit 3-1 is constituted by a transistor Q1, comparator CP1, reference resistor Ref. Zener diode ZD1, and resistors R1 and R2. A reference voltage Vref generated at the connection point between the Zener diode ZD1 and the reference resistor Ref is compared with a detection voltage Vpv generated at the connection point between the resistors R1 and R2. The comparator CP1 controls the current flowing between the collector and the emitter of the transistor Q1 such that the reference voltage Vref matches the detection voltage Vpv. Accordingly, a constant voltage of 8.5 V is generated between the lines LA and LB as the exciting voltage Vex.
The D/A circuit 3-2 is constituted by resistors R3, R4, and R5, capacitor C1, comparator CP2, and switch SW5. One terminal of the resistor R3 is connected to the connection point between the resistors R1 and R2. The other terminal of the resistor R3 is connected to one terminal of the resistor R4 through the switch SW5. The other terminal of the resistor R4 is connected to the line LB. One terminal of the resistor R4 is connected to the non-inverting input terminal of the comparator CP2 through the resistor R5. The capacitor C1 is connected between the line LB and the non-inverting input terminal of the comparator CP2.
The exciting current adjustment circuit 3-3 is constituted by a resistor R6, transistor Q2, and switches SW1 to SW4. The output terminal of the comparator CP2 of the D/A circuit 3-2 is connected to the base of the transistor Q2. The emitter of the transistor Q2 is connected to the line LB through the resistor R6 and also connected to the inverting input terminal of the comparator CP2. The collector of the transistor Q2 is connected to the line LA through the series connection circuit of the switches SW4 and SW1 and the series connection circuit of the switches SW3 and SW2. The exciting coil 2 is connected between a connection point P1 of the switches SW1 and SW4 and a connection point P2 of the switches SW2 and SW3.
The exciting current adjustment circuit 3-3 alternately turns on the switches SW1 and SW3 and the switches SW2 and SW4 at a predetermined period in accordance with a command from the flow measuring output circuit 6, thereby generating the rectangular-wave-shaped exciting current Iex whose polarity alternately switches. The D/A circuit 3-2 ON/OFF-controls the switch SW5 in accordance with a command from a CPU 6-4 of the flow measuring output circuit 6 to switch the value (peak value) of the exciting current Iex to multiple levels in accordance with the measurement value by the flow measuring output circuit 6, as shown in FIG. 6.
The flow measuring output circuit 6 is constituted by a signal electromotive force detection circuit 6-1, a sample-and-hold circuit 6-2, an A/D conversion circuit 6-3, the CPU 6-4, a D/A conversion circuit 6-5, a current adjustment circuit (CCS) 6-6, and a constant voltage circuit 6-7 which supplies a power supply voltage to these circuits.
The signal electromotive force detection circuit 6-1 detects a signal electromotive force obtained between the electrodes 4a and 4b by using the potential of the ground electrode 5 as a reference. The sample-and-hold circuit 6-2 samples and holds the value of the signal electromotive force detected by the signal electromotive force detection circuit 6-1 immediately before the polarity switches. The A/D conversion circuit 6-3 converts the signal electromotive force (analog value) output from the sample-and-hold circuit 6-2 into a digital value and sends the digital value to the CPU 6-4.
On the basis of the signal electromotive force from the A/D conversion circuit 6-3, the CPU 6-4 obtains the measurement value (0 to 100% value) and outputs the measurement value to the D/A conversion circuit 6-5. The D/A conversion circuit 6-5 converts the measurement value (digital value) from the CPU 6-4 into an analog value and sends the analog value to the current adjustment circuit 6-6. The current adjustment circuit 6-6 has a comparator CP3, transistor Q3, and resistor R7. By causing the comparator CP3 to adjust the base current of the transistor Q3, a current Iccs that flows between the collector and the emitter of the transistor Q3 is adjusted in accordance with the measurement value from the D/A conversion circuit 6-5.
In accordance with the measurement value obtained on the basis of the signal electromotive force from the A/D conversion circuit 6-3, the CPU 6-4 gives a command to the exciting circuit 3 such that the exciting current Iex is supplied to the exciting coil 2 in accordance with the relationship shown in FIG. 6. More specifically, the CPU 6-4 issues a command to the exciting current adjustment circuit 3-3 to alternately turn on the switches SW1 and SW3 and the switches SW2 and SW4, thereby supplying the rectangular-wave-shaped exciting current Iex whose polarity alternately switches to the exciting coil 2. The CPU 6-4 outputs a command to the D/A circuit 3-2 to ON/OFF-control the switch SW5 at a duty ratio (a duty ratio which is set stepwise in accordance with the measurement value) corresponding to the measurement value, thereby adjusting the voltage value to the non-inverting input terminal of the comparator CP2. Accordingly, the value of the current flowing to the transistor Q2, i.e., the value of the exciting current Iex flowing to the exciting coil 2 is adjusted.
In the two-wire electromagnetic flowmeter 100, the exciting circuit 3 and flow measuring output circuit 6 are connected in series between the power supply lines L1 and L2. The current that flows through the exciting circuit 3 flows into the flow measuring output circuit 6 and becomes the output current Iout that is returned to the DC power supply. 200. FIG. 7 shows the simplified circuit arrangement of the two-wire electromagnetic flowmeter 100.
For example, when the measurement value by the CPU 6-4 is a 0% value, the instruction value of the exciting current Iex of the exciting coil 2 is 3.5 mA. The exciting circuit 3 requires a current of 0.5 mA to cause the exciting voltage circuit 3-1 to generate the exciting voltage Vex or set the voltage value to the non-inverting input terminal of the comparator CP2. Hence, letting Ia (0.5 mA) be the current that flows on the side of the exciting voltage circuit 3-1 including the D/A circuit 3-2, a current I1 that flows through the exciting circuit 3 is given byI1=Ia+Iex=0.5 mA+3.5 mA=4 mA 
The current I1 of 4 mA flows into the flow measuring output circuit 6. Let Ib be the current that flows on the side of the constant voltage circuit 6-7 of the flow measuring output circuit 6. The current Ib must have a value of 3 mA to drive the CPU 6-4 and the like. For this reason, when the current Iccs that flows on the side of the transistor Q3 is adjusted to 1 mA, a current 12 (=Iccs+Ib) that flows through the flow measuring output circuit 6 is 4 mA. The current I1 that flows through the exciting circuit 3 equals the current 12 that flows through the flow measuring output circuit 6. Hence, the output current Iout returned to the DC power supply 200 is 4 mA.
When the measurement value by the CPU 6-4 is, e.g., a 10% value, the CPU 6-4 adjusts the current Iccs that flows on the side of the transistor Q3 to 2.6 mA to set the output current Iout to 5.6 mA (−4 mA+1.6 mA). In this case, the exciting current lex in the exciting circuit 3 is 3.5 mA. Hence, the current Ia that flows on the side of the exciting voltage circuit 3-1 including the D/A circuit 3-2 is 2.1 mA.
Next, the reason why the value of the exciting current Iex is switched to multiple levels in accordance with the measurement value will be described. The value of the exciting current Iex is switched to multiple levels in accordance with the measurement value by the CPU 6-4 on the basis of the relationship shown in FIG. 6. The scheme of switching the value of the exciting current Iex to multiple levels is called a multi-point excitation switching scheme. If the multi-point excitation switching scheme is not employed, and for example, if the value of the exciting current Iex is fixed to 3.5 mA, the magnetic flux density extending through the fluid is low, and the signal electromotive force obtained by the signal electromotive force detection circuit 6-1 is small. For this reason, the output largely fluctuates due to the influence of noise that is superposed on the electrodes 4a and 4b in accordance with the flow velocity. That is, since the ratio of noise contained in the signal electromotive force becomes high as the flow rate increases, the S/N ratio becomes low, and stable flow measurement cannot be executed.
Let e be the signal electromotive force obtained by the signal electromotive force detection circuit 6-1. The signal electromotive force e is given bye=k·B·v·D where k is a constant, D is the diameter of the measurement tube 1, v is the average flow velocity, and B is the generated magnetic flux density. The generated magnetic flux density B is proportional to the exciting current Iex. When the exciting current Iex is increased, the signal electromotive force e also becomes large even at the same flow velocity. In the conventional two-wire electromagnetic flowmeter 100, in accordance with the measurement value, i.e., when the output current (4 to 20 mA) corresponding to the measurement value increases, the exciting current Iex is switched to a large value by using the increase amount of the output current.
For example, when the measurement value is a 20% value, the value of the exciting current Iex is switched to 6.7 mA. More specifically, the output current lout corresponding to the 20% value is 7.2 mA. The exciting circuit 3 requires the current Ia of 0.5 mA. Hence, a current up to 6.7 mA can be supplied as the exciting current Iex. When the measurement value is a 40% value, the value of the exciting current Iex is switched to 9.9 mA. More specifically, the output current Iout corresponding to the 40% value is 10.4 mA. The exciting circuit 3 requires the current Ia of 0.5 mA. Hence, a current up to 9.9 mA can be supplied as the exciting current Iex.
In this way, when the signal electromotive force e and the S/N ratio are increased by switching the exciting current lex to a large value in accordance with the measurement value, stable flow measurement can be executed.
In the two-wire electromagnetic flowmeter 100, the external voltage Vs supplied from the DC power supply 200, i.e., the voltage Vs obtained by subtracting the voltage drop Iout×RL in the external load RL from the power supply voltage (DC 24 V) of the DC power supply 200 is divided to the exciting circuit 3 and flow measuring output circuit 6. For this reason, the exciting voltage Vex generated by the exciting voltage circuit 3-1 is as low as 8.5 V. The larger the value of the rectangular-wave-shaped exciting current Iex supplied to the exciting coil 2 becomes, the longer the rise time of the exciting current Iex becomes.
FIG. 8 shows the rising waveform obtained when the value of the exciting current Iex is switched to 3.5 mA, 6.7 mA, 9.9 mA, and 12 mA. When the value of the exciting current Iex is as small as 3.5 mA, the exciting current Iex immediately rises, as indicated by a waveform I in FIG. 8. However, since the exciting voltage Vex generated by the exciting voltage circuit 3-1 does not change, the rise time becomes long, as indicated by waveforms II, III, and IV in FIG. 8, as the value of the exciting current Iex increases to 6.7 mA, 9.9 mA, and 12 mA. Accordingly, a steady-state region (a flat waveform portion after the exciting current Iex reaches a predetermined value) ta immediately before the polarity switches becomes short.
The sample-and-hold circuit 6-2 samples and holds the value of the signal electromotive force e immediately before the polarity switches. For example, the signal electromotive force e during 5 ms immediately before the polarity of the signal electromotive force e switches is sampled, and its average value is held. When the value of the exciting current Iex is 12 ma, the steady-state region ta immediately before the polarity of the exciting current Iex switches is only about 5 ms long. The obtained value of the sampled signal electromotive force e is based on the marginally stabilized exciting current Iex.
However, if the value of the exciting current lex exceeds about 12 mA, the signal electromotive force e is sampled when the exciting current Iex is still changing. Accordingly, the flow measurement value contains an error due to, e.g., an eddy current generated in the electrodes 4a and 4b. For this reason, in the conventional two-wire electromagnetic flowmeter 100 that employs the multi-point excitation switching scheme, the limit value of the exciting current Iex that is set to multiple levels in accordance with the measurement value is set to about 12 mA. More specifically, the exciting voltage Vex is set to 8.5 V. The maximum value of the exciting current Iex is set to 12 mA. Power design is done such that the steady-state region ta corresponding to 5 ms or more can be ensured within the current range of Iex ˜3.5 to 12 mA.
However, the conventional two-wire electromagnetic flowmeter 100 assumes a condition that the value of the exciting current Iex is smaller than that of the current I (=In=Iout) supplied from the DC power supply 200 (I>Iex). For this reason, the exciting current lex is small in a low flow rate region. Flow measurement becomes unstable in the low flow rate region.
More specifically, when the value of the exciting current Iex is larger than the supplied current I, and for example, when the instruction value from the CPU 6-4 to the exciting circuit 3 at the supplied current I of 4 mA (measurement value: 0% value) is set to 4.8 mA, the exciting current adjustment circuit 3-3 controls the peak value of the exciting current Iex to 4.8 mA. On the other hand, the comparator CP1 of the exciting voltage circuit 3-1 compares the reference voltage Vref with the detection voltage Vpv and controls to keep the exciting voltage Vex at 8.5 V. When a current of several ten μA is present, the Zener diode ZD1 generates a constant voltage.
In this case, when the rising waveform of the exciting current Iex reaches almost I=4 mA, as shown in FIG. 9, power supply starts to be short. The current to the Zener diode ZD1 decreases. Hence, the exciting voltage Vex cannot hold 8.5 V and starts to drop. As a result, the rising waveform of the exciting current Iex is abruptly rounded almost after exceeding the supplied current I. Hence, the stable region ta corresponding to 5 ms cannot be ensured.
For the above reason, in the conventional two-wire electromagnetic flowmeter 100, the value of the exciting current Iex is made smaller than that of the current I supplied from the DC power supply 200. For example, in a low flow rate region of 0% to 20% value, the value of the exciting current Iex is as small as 3.5 mA, and the magnetic flux density extending through the fluid is low. Hence, the signal electromotive force obtained by the signal electromotive force detection circuit 6-1 is small, and flow measurement is unstable.